Japanese Laid-Open Patent Application Publication 2002-021879 discloses a multi-plate centrifugal clutch in which the clutch plates are in a contact state even when the engine is in a stopped state. The multi-plate centrifugal clutch described in Japanese Laid-Open Patent Application Publication 2002-021879 includes centrifugal weights, a pressure plate, and clutch plates (a first plate and a second plate). Contact surfaces of the respective pressure plate and clutch plate are parallel to each other, and the pressure plate presses the clutch plate by using the centrifugal force of the centrifugal weights. Further, in the pressure plate, cam surfaces holding the respective centrifugal weights are provided so that each of the cam surfaces is at an angle with a contact surface between the pressure plate and the clutch plate. More specifically, the pressing force exerted by the pressure plate against the clutch plate is determined depending on the positions of the cam surfaces of the respective centrifugal weights that have received the centrifugal force on the cam surfaces.
The multi-plate centrifugal clutch described in Japanese Laid-Open Patent Application Publication 2002-021879 is configured so that the centrifugal weights do not make contact with the pressure plate when the engine is in a stopped state. In the case where the centrifugal weights are in contact with the pressure plate, there would be contact between the pressure plate and the clutch plate. In the case where the pressure plate and the clutch plate are in contact with each other, there occurs a drag state, which is similar to the engaged state of the clutch. In the case of a motorcycle using such a multi-plate centrifugal clutch causing the drag state, there arises a problem in which a travel resistance occurs if the motorcycle with the engine in the stopped state is pushed and moved. Hence, in the multi-plate centrifugal clutch described in Japanese Laid-Open Patent Application Publication 2002-021879, the cam surfaces provided on the pressure plate are each formed into such a shape that the centrifugal weights separate a predetermined distance from the pressure plate when the engine is in a stopped state.
However, in the case of the motorcycle including the multi-plate centrifugal clutch described in Japanese Laid-Open Patent Application Publication 2002-021879, while the drag state of the clutch is prevented, a situation can arise when parking the motorcycle, for example, on a sloped road, in which the motorcycle resistance against the sloped road is insufficient to prevent the motorcycle from rolling.
Further, in the case where the motorcycle according to the Japanese Laid-Open Patent Application Publication 2002-021879 includes an auto-controlled gear transmission, i.e., a so-called AMT (automated manual transmission), when the engine of the motorcycle is in the stopped state, the driving state is shifted to the neutral position, that is, the gear input operation is not effected. Hence, in the case where the motorcycle according to Japanese Laid-Open Patent Application Publication 2002-021879 is a motorcycle including the AMT, the motorcycle cannot be parked stably when parking, for example, on a sloped road. Hence, in the case where the motorcycle according to Japanese Laid-Open Patent Application Publication 2002-021879 includes the AMT, dedicated brake means is necessary to park the motorcycle stably on a sloped road. On the other hand, however, in the case where the motorcycle according to Japanese Laid-Open Patent Application Publication 2002-021879 is a motorcycle in which the dedicated brake means is not provided, application of the transmission including the clutch according to Japanese Laid-Open Patent Application Publication 2002-021879 is limited to motorcycles including a gear transmission that is not controlled automatically, i.e., a so-called MT (manual transmission).